Developing Web apps is a sucky experience because when things don't work, you rarely get meaningful feedback. If you're lucky, you get a message in the JS console with the function that failed, the source line, and an nsresult to explain why (usually the ever-helpful "NS_ERROR_FAILURE"). It shouldn't be terribly hard to fix this; the code that detects and returns NS_ERROR_FAILURE can and should export a meaningingful error message. This would improve our own lives and the lives of Web developers everywhere. See http://wiki.mozilla.org/Gecko:ErrorMessages for more discussion.
I think this is a project where contributors without elite C++ coding skillz could make a huge difference.
Ben, your comment doesn't seem particularly related to this bug (which is a technical issue), and more importantly, most people involved with documentation will not see your comments. I suggest that you post to the MDC mailing list: http://developer.mozilla.org/en/docs/MDC:Community (Note that there is an ongoing discussion about making it easier for contributors to get started.)
This report needs a small test case.
Having looked over the bug, I don't believe this is good g-f-b fodder in its current state. At minimum, this bug would need a mentor who would be on the hook to provide a spec from which to start and pointers towards some relevant places that could benefit from these new error messages. Not every location has to be identified up front, but a sampling from which a contributor could jump off from would go a long way to making this bug actionable.
Whiteboard: [good first bug]
Component: DOM → Canvas: 2D
Summary: Lack of DOM error message sucks → Improve error reporting in CanvasRenderingContext2D.cpp
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